This is an application to renew a grant to study RNA synthesis in minus-strand RNA viruses. Nonsegmented negative-strand (NNS) RNA viruses include some of the most significant human pathogens that are a major ongoing threat to US public health. To combat those agents, we need a combination of antiviral drugs and vaccines. Our long-term objective is to understand the mechanisms by which the replication machinery of vesicular stomatitis virus (VSV), a prototype of the NNS RNA viruses, functions. VSV is the ideal choice for such studies because it is the only NNS RNA virus for which robust transcription can be reconstituted in vitro from purified recombinant components. The catalytic core of the RNA synthesis machinery is a 241 kDa large polymerase protein (L) that contains an RNA dependent RNA polymerase (RdRP), a polyribonucleotidyltransferase (PRNTase) that caps the mRNA, and a dual specificity mRNA cap methyltransferase (MTase). During mRNA synthesis, those activities are coordinated so that the nascent mRNA is capped, methylated and polyadenylated. Although L contains all the enzymatic activities for RNA synthesis, it requires a 29 kDa phosphoprotein (P) that bridges interactions between L and the nucleocapsid protein (N) that completely coats the genomic RNA template. In the last grant period, we developed in vitro assays to separately study each of the steps of mRNA cap addition independent of ongoing transcription. Those assays, combined with a powerful reverse genetic system allow mechanistic analysis of lethal mutations in the viral RNA synthesis machinery. We have used those assays to provide a map of where the different enzymatic activities for each step of mRNA cap addition are localized within L. Those studies lead us to the hypothesis that L contains independent functional domains whose activities are coordinated by the assembly of L into the RNA synthesis machine of the L-P complex with the N-RNA template. A major gap to understanding the mechanisms by which the RNA synthesis machinery of NNS RNA viruses function is the absence of structural information for L. During the next funding period, we will use electron microscopy, X-ray crystallography and in vitro assays of polymerase function to provide unique structural and functional insights into the RNA synthesis machinery of VSV. We will: (i) determine the functional organization of the VSV polymerase complex;(ii) determine the three dimensional structure of the VSV polymerase, and (iii) probe the relationship between the mRNA capping and RNA synthesis activities of L. The successful completion of this study will provide a structure of the polymerase of an NNS RNA virus as well as new mechanistic insights into the function of this RNA synthesis machine that may help in the rational design of antiviral therapeutics and candidate vaccines.